1. Field of the Invention
This invention relates to the production of hydrocarbons from subsurface formations and more particularly to a system and process for preventing plugging of formation fluid passageways caused by the accumulation of insoluble salts precipitates resulting from the interaction between precipitate precursor ions in injection water and resident cations in the subsurface formations.
2. Description of the Related Art
Hydrocarbons, such as oil and gas, are recovered from the earth's subsurface formations through production wellbores that penetrate hydrocarbon-bearing formations or reservoirs. Perforations are made from the production wellbore to the formation to facilitate flow of the hydrocarbons from the hydrocarbon-bearing formations to the production wellbores. Water is sometimes injected under pressure into injection zones formed in the subsurface formations to stimulate hydrocarbon production through the production wells in a field.
Water is injected by itself as a component of miscible or immiscible displacement fluids. Sea water (for offshore wells) and brine produced from the same or nearby formations (for onshore wells) are most commonly used as the water source. Such water usually contains large amounts (concentration) of precursor ions, such as divalent sulfate (SO.sub.4.sup.-), which form insoluble salts when they come in contact with cations, such as Ba.sup.++, Sr.sup.++ and C.sup.++, resident in the formations. The resulting salts (BaSO.sub.4, SrSO.sub.4 and CaSO.sub.4) can be relatively insoluble at subsurface formation temperature and pressure. Consequently, such salts precipitate out of the solution. The precipitation of the insoluble salts accumulates and consequently plugs the subsurface fluid passageways. The plugging effects are most severe in passageways in the formation near the injection wells and at the perforations of the production wells. Solubility of the insoluble salts further decreases as the injection water is produced to the surface through the production wells, due to the reduction of the temperature and pressure as the fluids move to the surface through the production wells.
For the purpose of this invention, subsurface or formation fluid passageways includes pores in the formation matrix, fractures, voids, cavities, vugs, perforations and fluid passages through the wells, including cased and uncased wells, tubings and other fluid paths in the wells. The term precipitates means insoluble salts, crystals or scale. The term plugging as used herein means reduction in the porosity and/or permeability of fluid passageways. The term injection water as used herein means any fluid containing water that is injected into a subsurface formation to facilitate recovery of hydrocarbons from subsurface formations.
Mechanical methods, such as passing the untreated water through a nano-filtration membrane, have been used to remove substantial amounts of the precursor ions from the water at the surface before injecting it into the wellbore. Sea water typically contains between 2700 to 2800 ppm of divalent SO.sub.4.sup.-. The nano-filtration membrane process usually reduces this concentration to between 50 and 150 ppm. Under many subsurface reservoir conditions, such a concentration of the precursor ions produces sufficient amounts of the insoluble salts to plug or seal the formation fluid passageways. Thus, the filtration of the injection water alone by nanofilters, in many cases, is not sufficient to prevent plugging of the subsurface formation passageways.
Chemicals or additives alone are often injected into the untreated water to inhibit the in-situ growth of crystals from insoluble salt precipitation. A variety of additives are injected into the injection water at the surface or directly into an injection well. Production wells are also often treated with back-flow of fresh brine containing additives to prevent plugging of the passageways. Use of chemicals alone can be cost-prohibitive, environmentally unfriendly, and not totally effective, especially when high concentrations of naturally-occurring cations are present in the subsurface formation.
The present invention provides a system and process which simultaneously utilize both a mechanical process and a chemical process to effectively control or prevent in-situ growth of crystals in the subsurface formation, thereby preventing the plugging of the formation fluid passageways.